Disc drives read and write information along concentric tracks formed on discs. Each of the concentric tracks is divided into a plurality of sectors. Each of these sectors usually includes a servo field and a data field that are contiguous. To locate a particular track on a disc, disc drives typically use the embedded servo fields in the sectors. The embedded servo fields are utilized by a servo sub-system to position a head over a particular track on a rotating disc. The servo field of each sector includes a sector timing mark to verify receipt of the sector and to establish the timing between sequential sectors.
In current disc drives, the servo fields are written onto the disc in-situ (i.e., after the disc is mounted on the spindle motor of a disc drive) when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position during operation. Since each track has a different linear velocity from every other track on the disc, signals written at a constant frequency do not exhibit a constant data density from track to track. For instance, a servo field written at a constant frequency in a sector of an inner track will occupy less linear distance on the inner track than a constant frequency servo field written on a more radially distant outer track. Nevertheless, servo fields are written using constant frequency signals across the tracks. In such a case, radially aligned servo fields occupy different linear distances in their respective tracks, but radially aligned servo fields occupy an identical time window in each track (the angular velocity of each track is identical). Further, in in-situ written tracks, the radial distance between the spin axis of the spindle motor and each sector of a particular track is substantially equal. Thus, the time that elapses between the detection of timing marks of any two adjacent sectors of an in-situ written track, by a head positioned over the track, is substantially constant. This elapsed time between two adjacent sector timing marks is referred to as sample rate, which is substantially constant for a drive employing in-situ written discs. Consequently, a timing mark search window, which is relatively narrow and which is generated at constant intervals of time (equal to the constant sample rate), is used to detect timing marks in drives with such in-situ written discs.
To meet the demand for greater recording density in disc drives, track writing is undergoing a fundamental change. In the near future, manufactured disc drives will include discs with tracks that are pre-written onto the discs before the discs are mounted on the spindle motor of the drive. When such discs with pre-written tracks (pre-written discs) are mounted on a spindle motor of a disc drive, there is a certain amount of misalignment of the track center of the disc and the spin axis of the spindle motor. Because of such disc mounting tolerances, the tracks will be eccentric to the spin axis of the spindle motor. Radial distances between the spin axis of the spindle motor and different sectors on the same track will differ, thereby causing variations in sample rate between timing marks of different adjacent sectors on the same track during disc drive operation. In drives with pre-written discs, if a timing mark search window is generated at a constant or nominal time interval, as in the case of drives including in-situ written discs, the timing window will have to be relatively wide to account for variation in sample rate is these drives with pre-written discs. Widening the timing mark search window is undesirable since it increases the opportunity of false reads of the timing marks. Additionally, in drives that utilize embedded spindle motor control, variations in sample rate complicates the spindle motor control since the spindle motor controller is driven by a timing error that is related to variations in sample rate.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.